Magnetic recording media are widely used as audio tapes, video tapes or as floppy disks. A magnetic recording medium is fundamentally composed of a nonmagnetic support having laminated thereon a magnetic layer containing ferromagnetic particles dispersed in a binder.
A magnetic recording medium requires high levels of quality with regard to various characteristics such as electromagnetic characteristics, running durability and running performance. In fact, even higher levels of quality is demanded for reproducing original sounds from an audio tape. Also, there is a high demand for excellent electromagnetic characteristics for reproducing original images for a video tape.
As described above, it is not enough to provide excellent electromagnetic characteristics for a magnetic recording medium; good running durability is also required. And an abrasive agent plays an important role in the achievement of good running durability. That is, abrasive agents contained in a magnetic layer are distributed wholly in the magnetic layer, and a certain concentration of these abrasive agents is also present on the surface of a magnetic layer. When a magnetic recording medium runs, contacting parts of running system such as a head, the abrasive agents present on the surface of the magnetic layer exhibit an abrasive effect at the contact surface between the surface of the magnetic layer and the magnetic head. Accordingly, incorporation of abrasive agents into a magnetic recording medium improves running durability. However, when the abrasive agents present on the surface of the magnetic layer represent only a small percentage of all the abrasive agents added into the magnetic layer, it is difficult to secure sufficiently excellent running durability. For example, when an additive amount of abrasive agents is increased to improve durability, the concentration of ferromagnetic particles decreases, and when abrasive agents having a large particle diameter are used, abrasive agents are likely to excessively extrude from the surface of the magnetic layer. And as a result thereof, the above-described electromagnetic characteristics deteriorate. Such deterioration is a serious problem.
In order to solve the above problem, it was proposed that running durability could be wholly improved by providing two magnetic layers, i.e., an upper magnetic layer and an under magnetic layer, whereby the upper layer is excellent particularly in running durability and the under layer is excellent particularly in electromagnetic characteristics (as described in JP-A-58-200425) (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"). That is, additives such as abrasive agents are not added into the under layer so that the content of ferromagnetic particles is made high to improve electromagnetic characteristics. At the same time, running durability could be improved without deteriorating electromagnetic characteristics by (1) defining the thickness of the upper layer as from 0.5 to 1.5 .mu.m, (2) adding abrasive particles into the upper layer only, and (3) using abrasive agents having a maximum particle diameter not exceeding the thickness of the upper layer.
Thus, a magnetic recording medium having the simultaneous advantage of good electromagnetic characteristics and good running durability could be obtained to some extent by providing two magnetic layers. However, it cannot be said that the above simultaneous advantage is completely excellent. In particular, the quality of running durability may suffer.